This invention relates to tamper-resistant fasteners. Fasteners which resist removal by conventional screwdrivers and other means are known to the art. One method of inhibiting the unauthorized removal of fasteners has been to construct a fastener head having two sockets, one of which is eccentrically disposed relative to the other. The fastener can then be turned only upon the application of a special driving tool, which matches the shape of the sockets in the fastener head. An example of this type of fastener is disclosed in U.S. Pat. No. 2,338,023, to Bugg, in which two such sockets are embedded in a fastener head. The eccentricity of one socket permits the turning of the fastener by a driving tool having projections matching the sockets of the fastener. Since the sockets are typically of circular shape, they resist efforts to remove the fasteners by ordinary means.
Fasteners having sockets similar to those described above, known in the art as "specialty head" fasteners, present problems in operation. When torque is applied to a conventional fastener head having two sockets, there is a marked tendency for the driving tool to tilt considerably as torque is being applied. The tilting effect interferes greatly with the installation and removal of such fasteners, since the tilting diverts much energy from the user, as attempts are made to keep the tool in a straight position.
Conventional tamper-resistant fasteners of the two-socket variety also do not provide an ideal fit between fastener head and driving tool. With only two sockets, the connection between head and tool is not particularly strong, and the tool must be held very firmly against the fastener head while torque is applied.
Other types of tamper-resistant fasteners, which do not create tilting problems, include those containing a one-way head, and those having a breakaway head. The former type can be driven in only one direction. The latter type is driven by conventional tools, but can be used only once, because part of the head breaks away from the fastener after use. The specialty-head fastener is the only one of the three types that can be screwed and unscrewed many times.